1.5.7

1. April 2022 exoplanet update
2. Use web browser for add-on list and news archive
3. Rearrange some setting options
4. Fix brightness issue on rendering nebulae and other objects

celestiamobile/Celestia@5a12452
celestiamobile/CelestiaCore@4342818
CelestiaProject/CelestiaContent@7f816cc
celestiamobile/CelestiaLocalization@d569a80